Pope John Paul II has suffered a cardiac arrest and is in a grave condition, the Vatican has announced.
The urgent communique came hours after the Pope's health worsened with a high fever caused by an infection.
"Following a urinary tract infection, septic shock and a cardiocirculatory collapse occurred," Vatican spokesman Navarro-Valls said in his statement.
On Thursday he received the Saint Viaticum, the Catholics' last rites for the sick and dying, the spokesman said.
This is a sign that his closest advisers fear that the end of one of the longest papal reigns is fast approaching, says the BBC's David Willey in Rome.
And for the first time during the Pope's current illness the mood at the Vatican has suddenly shifted to one of pessimism, our correspondent says.
An update on the Pope's health is expected later on Friday morning.
At the Vatican, Cardinal Camillo Ruini, the Pope's deputy for Rome who is traditionally charged with breaking the news of a papal death, called on believers to "intensify their prayers".
Pope stays put
The pontiff was provided with "all the appropriate therapeutic provisions and cardio-respiratory assistance", Mr Navarro-Vals' statement early on Friday said.
"This morning the condition of the Holy Father is very serious," the statement said, but the 84-year-old pontiff remained "conscious, lucid and tranquil", the spokesman said.
He said the Pope had decided not to return to Rome's Gemelli hospital - but is being treated in his apartment by a team of four top consultants from the Catholic teaching hospital in Rome and his private doctor Renato Buzzonetti.
Groups of faithful have gathered at St Peter's Square in Rome to pray for the Pope's speedy recovery.
On Thursday afternoon, the Pope's temperature soared to 40C (104F), Italian media reported.
He developed breathing troubles and has difficulty in swallowing as a result of the progress of Parkinson's Disease, an incurable condition from which he has been suffering for nearly a decade.
Prelates are openly expressing pessimism about the possibility of the Pope ever resuming the guidance of his one billion-strong church, our correspondent says.
The pontiff is being fed through a nasal tube to aid his recovery from throat surgery last month.
The Pope had appeared briefly at the window of his Vatican apartment on Easter Sunday to bless the faithful, but was not able to speak.
It was the first time during his 26-year pontificate that the Pope delegated the main Easter ceremonies to his cardinals.
He tried again to speak to the faithful a few days later - a sign of his extraordinary strong will, our correspondent says.
So far this year the Pope has had two spells in hospital where he received treatment for breathing problems and underwent an operation on his throat
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Shame actually, one of the best popes we've had, new guy will probs be a wanker.
